Metabolic substrates, muscle fibre composition and fibre size in late walking and normal children.

In 10 children with delayed walking, muscle specimens were obtained by needle biopsy from the vastus lateralis muscle for determination of ATP, creatine phosphate (CP), glycogen and lactate concentration, as well as fibre composition and fibre size. The values obtained are compared with corresponding results for a control group of 25 children aged 2 months--11 years. Details of the control series are given and its validity as a reference group is discussed. Six children (group I) who showed minor gross motor delay but otherwise normal development at follow-up were found to have lowered ATP, CP and glycogen concentrations, compared to 4 children (group II) who had general developmental delay, and to the control group. The children in group I were shown to have smaller fibre size and a greater difference between type 1 and 2 fibres than the children in group II and in the control group. No differences in the concentrations of metabolic substrates or the morphometric values could be detected between group II and the control group.